Field of the Invention
The invention is directed to a light-emitting device and more particularly, to a semiconductor light-emitting device.
Description of Related Art
With the evolution of photoelectrical technology, traditional incandescent bulbs and fluorescent lamps have been gradually replaced by solid-state light sources of the new generation, such as light-emitting diodes (LEDs). The LEDs have advantages, such as long lifespans, small sizes, high shock resistance, high light efficiency and low power consumption and thus, have been widely adopted as light sources in applications including household lighting appliances as well as various types of equipment. Besides being widely adopted in light sources of backlight modules of liquid crystal displays (LCDs) and household lighting appliances, the application of the LEDs have been expanded to street lighting, large outdoor billboards, traffic lights and the related fields in recent years. As a result, the LEDs have been developed as the light sources featuring economic power consumption and environmental protection.
Recently, most developers of solid-state light sources make effort to pursue good luminance efficiency. Subjects with respect to improving the luminance efficiency of the LEDs are generally divided into how to improve internal quantum efficiency (i.e., luminance efficiency of a light-emitting layer) and how to improve external quantum efficiency (which is further affected by light extraction efficiency). However, in a conventional gallium nitride (GaN) LED, a band gap between a P-type GaN semiconductor layer and an N-type GaN semiconductor layer is approximate to a band gap of the light-emitting layer, such that blue light or ultraviolet (UV) light emitted from the light-emitting layer is easily absorbed, which leads to reduced luminance efficiency of the LED.